1. Field of the Invention
The present invention relates to a process for producing a sintered body of a metal boride and also to a raw material composition therefor.
2. Description of the Related Art
Sintered bodies of metal borides such as TiB.sub.2 and ZrB.sub.2 find use as wear-resistant elements, cutting tools, corrosion-resistant refractory materials, etc. owing to their characteristic properties such as high hardness, high strength, and high electrical conductivity.
Sintered bodies of metal borides are usually produced from a metal boride powder containing a sintering auxiliary by hot press sintering. For example, Watanabe et al. obtained a nearly voidless TiB.sub.2 sintered body from a TiB.sub.2 powder containing 1% CoB by hot-pressing at 1700.degree. C. for 30 min. (See "Funtai Oyobi Funmatsu Yakin" (J. Jap. Soc. Powder and Powder Metallurgy), vol. 33 (1986), p. 38.)
Although there have been proposed many sintering auxiliaries such as FeB, Ni.sub.4 B.sub.3, and NiB for TiB.sub.2, they impair the inherent properties of TiB.sub.2. For example, they increase the coefficient of thermal expansion and density and decrease hardness and thermal conductivity.
For an ordinary commercial powder to be compacted without the aid of an sintering auxiliary, heating under a very high pressure is required. In fact, a compact sintered body of HfB.sub.2 or ZrB.sub.2 is obtained by hot pressing at 1800.degree. C. and 120000 psi (ca. 0.8 GPa). (See J. Am. Ceram. Soc., vol. 52, No. 1, pp. 30-36.) A disadvantage of this process using such a high mechanical pressure is that it can be applied only to the production of small sintered bodies of simple shape and cannot be applied to the production of sintered bodies of practical use. It has also been reported that the hot pressing of ZrB.sub.2 requires a high pressure in excess of 40 MPa at 2050.degree. C. if it is to have a density higher than 95%. (See Hayami et al., "Yogyo Kyokaishi" (Journal of Ceramics Industry Society), vol. 86, No. 2 (1978), pp. 352-359.)
There has been reported a process for producing a TiB.sub.2 sintered body by sintering under normal pressure a TiB.sub.2 powder of ultrafine particles having a specific surface area as high as 7 m.sup.2 /g which is produced by gas phase reaction in a plasma arc. (See J. Am. Ceram. Soc., 67 (1984), p. 207.) A disadvantage of this process is that such a fine powder is so pyrophoric that it should be handled in an inert gas atmosphere, which adversely affects the productivity.
On the other hand, there is disclosed in Japanese Patent Laid-open No. 162289/1984 a process for producing a compact TiB.sub.2 sintered body by uniformly mixing a titanium-containing powder, a boron-containing powder, and a carbon powder, keeping the mixture at 1600.degree.-2200.degree. C. for 5-45 minutes, and then keeping the mixture at 2250.degree.-2600.degree. C. for 10-60 minutes.
A disadvantage of this process is that it yields a TiB.sub.2 sintered body containing a large amount of residual carbon which leads to the following shortcomings.
(a) The sintered body has a low hardness and strength on account of the residual carbon. PA0 (b) The sintered body is poor in oxidation resistance because the residual carbon burns at high temperatures. PA0 (c) In addition, the abundant carbon in the raw material makes it difficult to produce a uniform mixture when the raw materials are dry-mixed. In the case where the raw materials are wet-mixed, the segregation of carbon is liable to take place during drying because carbon particles easily move in the slurry. PA0 A. A compressed body of raw materials is heated until carbon monoxide is generated and a porous body is formed. PA0 B. Raw materials are heated in a vacuum or in an inert atmosphere below a temperature at which the formation of a metal boride takes place. The resulting materials are crushed uniformly and then molded by pressing or pelleting. Finally, they are reacted to form a metal boride.
Japanese Patent Laid-Open No. 169983/1984 refers to a formation of TiB.sub.2 by reacting TiO.sub.2, TiC and B.sub.4 C. According to the inventor's experiments, however, the sintering of a metal oxide, a metal carbide and boron carbide mixed in the stoichiometric ratio do not result in the formation of a metal boride having a density higher than 90% of the theoretical density. This is probably due to shortage of boron which is caused because part of boron is evaporated in the form of boron oxide from the Ti-B-O compound produced during the sintering.
The metal boride in this publication can be obtained as follows.
These processes A and B fail to provide a sintered body having high density. Further, the process B requires a step of taking the once synthesized metal boride out of the sintering furnace, followed by the step of crushing. This causes serious contamination of the powder and adversely affects the degree of sintering. The resultant metal boride contains interconnected pores in the amount as large as 10-45 vol %. Such a porous metal boride cannot find use as high-performance mechanical components.